Room 1: A button, a door triggered open by the button, and an exit on the other side.

Solution: Straightforward. One person has to stay behind. The trick is persuading which volunteer to do so.

Room 2: A button, a door triggered open by the button, and an exit on the other side. A pile of pistols.

Solution: As per room 1. Alternately, you can kill whomever elects to stand on the button first. This solution is a little non-obvious - in the Source engine you cannot pick up or move bodies.

Room 3: A button, a corridor with two doors both trigged open by the button, and the exit on the other side of the second door. A timer button on a pedestal in the first room triggers a turret in the corridor which kills any one who has crossed through the first door and hasn't made it to the second door.

Solution: Again, as per room 1. The 'dumb sap' sitting on the button can get off at any point, trapping people between the two doors though. He can then wander over and activate the pedestal button and kill them at his leisure. Alternately, any one else can spam the corridor with turret fire. So, who do you trust?

Room 4: A button, a door triggered open by the button, and an exit on the other side. There's a weighted companion cube next to the exit. There's a rack of rocket launchers next to the cube.

Solution: Straightforward. One person has to stay behind. The trick is persuading which volunteer to do so. Usually you can do this by saying 'I'll bring the cube back for you' and then skipping by it, whistling, as you run out the exit. The rocket launchers are for killing anyone who is dumb enough to try to take the companion cube back.

Room 5: An impassable pit of water with a platform that moves backwards and forwards to allow you to cross it. The platform is stationary on your side. A button on your side moves the platform to the other side. A button on the other side moves it back.

Solution: Different tools. Same game.

Any other suggestions? I'm sure there are elements of the game that can be built up to set up Prisoner's Dilemma type solutions, which are a little more subtle than this straightforward killing off process I've outlined. I also like the idea of having a limited number of lives (Clones in Paranoia parlance) which you can come back as and take your revenge with.

[Edit: I've come up with a complete design. Now I need help. See part two for details.]